This invention relates to a semiconductor device having a heat sink attached to a lead frame and a method for making the same. More particularly, the present invention relates to a method and structure for attaching a heat spreader or heat slug to a lead frame.
FIG. 1a is a top view of a semiconductor device 100 which includes semiconductor die 101, die attach adhesive 102, heat slug 104, bonding wires 105-106, molding compound 107, lead frame 108 (which includes leads 111 and 112) and double-sided polyimide tape 109. Elements which are encased in molding compound 107 are shown in dashed lines in FIG. 1a. FIG. 1b is a cross sectional view of semiconductor device 100 along plane 1bxe2x80x941b of FIG. 1a.
Heat slug 104 is used to dissipate heat generated within device 100 during operation of device 100. Heat generated within semiconductor die 101 is conducted through die attach adhesive 102 to heat slug 104. Heat slug 104 is typically a metal, such as copper or aluminum, which has a high thermal conductivity. Heat slug 104 conducts heat to the environment external to device 100 and thereby prevents heat build-up in the vicinity of die 101. As illustrated in FIG. 1b, the bottom portion of heat slug 104 extends through molding compound 107 such that this portion is exposed to the outside environment. A heat spreader (not shown) is similar to heat slug 104, except that a heat spreader is totally encapsulated by molding compound 107 (i.e., is not exposed to the outside environment). Heat spreaders and heat slugs are hereinafter generically referred to as heat sinks.
Polyimide tape 109 is used to connect heat sink 104 to lead frame 108. Polyimide tape 109 is typically positioned around the perimeter of the upper surface of heat sink 104. The leads of lead frame 108 (including leads 111 and 112) are positioned on polyimide tape 109, such that polyimide tape connects heat sink 104 and lead frame 108. After heat sink 104 is connected to lead frame 108, die 101 is attached to central area of the upper surface of heat sink 104 using die attach adhesive 102. Polyimide tape 109 holds heat sink 104 and lead frame 108 in a fixed relationship when molding compound 107 is formed around the various elements of semiconductor device 100.
FIG. 2 is a cross sectional view of tape 109, which includes polyimide layer 201 and adhesive layers 202 and 203. Polyimide tape 109 is thermally conductive and electrically insulating. As a result, heat received by heat sink 104 is conducted through polyimide tape 109 to lead frame 108. However, because polyimide tape 109 is electrically insulating, polyimide tape 109 does not short the various leads of lead frame 108.
To attach heat sink 104 to lead frame 108, adhesive layer 202 is placed on heat sink 104, thereby fixing polyimide tape 109 to heat sink 104. Lead frame 108 is then positioned on adhesive layer 203, thereby fixing polyimide tape 109 to lead frame 108. Lead frame 108 and heat sink 104 are then clamped together and polyimide tape 109 is heated, thereby curing adhesive layers 202 and 203 and attaching lead frame 108 to heat sink 104. Lead frame 108 must be placed on adhesive layer 203 shortly after adhesive layer 202 is placed on heat sink 104 to prevent adhesive layer 203 from becoming contaminated by exposure to the environment.
Polyimide tape 109 is relatively expensive because adhesive layers 202 and 203 must be laminated onto each side of polyimide layer 201. Moreover, if polyimide tape 109 is improperly clamped between heat sink 104 and lead frame 108, gaps may exist between adhesive layer 202 and heat sink 104 and/or between adhesive layer 203 and lead frame 108. As a result, poor adhesion may exist between heat sink 104 and lead frame 108. Further contributing to this problem, polyimide tape 109 has a tendency to warp, thereby promoting a sub-optimal connection between lead frame 108 and heat sink 104. Additionally, polyimide layer 201 must have a minimum thickness, typically more than one mil, to form a film. As polyimide layer 201 becomes thicker, the ability of polyimide tape 109 to conduct heat is reduced.
U.S. Pat. No. 4,783,428 issued to Kalfus discloses another method for coupling a lead frame to a heat sink. In Kalfus, a first layer of thermally conductive epoxy is screen printed onto a heat sink. The first layer of epoxy is cured and a second layer of thermally conductive epoxy is screen printed on the first layer of epoxy. A lead frame is then placed on the second layer of epoxy and the second layer of epoxy is cured to attach the lead frame to the heat sink. The lead frame must be connected to the heat sink soon after the second layer of epoxy is screen printed to prevent contamination of the second layer of epoxy.
It would therefore be desirable to have a method and structure for attaching a lead frame to a heat sink which overcomes the above described short-comings of polyimide tape. It would also be desirable if this method and structure provided a heat sink which (1) includes the materials necessary to connect the heat sink to the lead frame, and (2) can be conveniently stored for long periods of time until the heat sink is to be connected to the lead frame.
Accordingly, the present invention provides a semiconductor device which includes a heat sink and a lead frame. A fully cured layer of thermally conductive epoxy is connected to the heat sink and a layer of thermoplastic is connected to the fully cured epoxy layer. The lead frame is connected to the layer of thermoplastic. Both the epoxy layer and the thermoplastic layer are applied in paste form, thereby substantially eliminating the adhesion problems previously associated with the use of polyimide tape 109.
The epoxy and thermoplastic layers can be formed in a variety of ways. In one embodiment, the fully cured epoxy layer is formed on the heat sink and the thermoplastic layer is formed over the fully cured epoxy layer. The resulting structure can advantageously be stored until the heat sink is connected to the lead frame. In another example, the fully cured epoxy layer is formed on the heat sink and the thermoplastic layer is formed on the lead frame. Again, the resulting structures can be stored (separately) until the heat sink is to be connected to the lead frame.
In another embodiment, the fully cured epoxy layer is connected to the lead frame (instead of the heat sink) and the thermoplastic layer is connected to the heat sink (instead of the lead frame). Again, the epoxy layer is connected to the thermoplastic layer. In this embodiment, the epoxy and thermoplastic layers can be formed in a variety of different ways. In one example, the fully cured epoxy layer is formed on the lead frame and the thermoplastic layer is formed over the fully cured epoxy layer. Alternatively, the fully cured epoxy layer is formed on the lead frame and the thermoplastic layer is formed on the heat sink. Again, the resulting structures can be stored until the heat sink is to be connected to the lead frame.
In accordance with another embodiment of the invention, a heat sink structure includes a thermally conductive heat sink, a fully cured first layer of thermally conductive epoxy located over the heat sink, and a partially cured second layer of thermally conductive B-stage epoxy located over the first layer of epoxy. The first and second layers of epoxy are applied in paste form and the resulting heat sink structure can be stored until the heat sink structure is connected to a lead frame. When the heat sink structure is to be connected to the lead frame, these structures are positioned such that the partially cured second epoxy layer is contacting the lead frame. The second epoxy layer is then fully cured, thereby connecting the heat sink structure to the lead frame.
In a variation, the fully cured epoxy layer is formed on the heat sink and the partially cured epoxy layer is formed on the lead frame. In another variation, the fully cured epoxy layer is formed on the lead frame (rather than the heat sink) and the partially cured epoxy layer is formed on the fully cured epoxy layer. In yet another variation, the fully cured epoxy layer is formed on the lead frame and the partially cured epoxy layer is formed on the heat sink.
The present invention will be more fully understood in light of the following detailed description taken together with the drawings.